Field
Apparatuses and methods consistent with exemplary embodiments relate to a resonant converter, a power supply and a power controlling method thereof, and more particularly, to a resonant converter, a power supply and a power controlling method thereof which operates under zero voltage switching (ZVS).
Description of the Related Art
An electronic apparatus including a display apparatus, such as a television (TV) includes a switching mode power supply (SMPS) as a power supply which supplies power necessary for its operation. The SMPS may include, e.g., an LLC resonant converter which supplies power to a main display of a display apparatus.
Efforts made by SMPS to obtain high power density are limited by the size of a passive element. High frequency operation may significantly reduce the size of the passive element, such as a transformer or a filter, but may result in switching loss. To reduce switching loss and ensure the high frequency operation, a resonant switching technology has emerged.
FIG. 1 illustrates an LLC resonant converter of the related art.
As shown in FIG. 1, the LLC resonant converter includes a square wave generator 11, a resonant part 13 and a rectifier 15.
The square wave generator 11 operates by a control pulse frequency modulation (PFM) integrated circuit (IC) which changes a frequency depending on a load state, and may be implemented as a half-bridge or a full bridge type.
FIG. 2 illustrates an example of the square wave generator 11 of the LLC resonant converter which is implemented as a half-bridge type. FIG. 3 illustrates an operating waveform of the LLC resonant converter in FIG. 2.
Switches Q1 and Q2 of the square wave generator 11 are alternately turned on and off at a duty ratio of 50% depending on the frequency that changes according to load. The switches Q1 and Q2 have a small dead time during their alternate operation, as in FIG. 3, and perform continuous operations without arm short. The resonant part 13 includes a capacitor Cr, a leakage inductance Lr and a magnetizing inductance Lm of a transformer.
The rectifier 15 rectifies an alternating current (AC) with a rectifying diode and a capacitor to generate a direct current (DC) voltage. The rectifier 15 may be formed by a combination of a full-bridge diode or a center-tap and a capacitor.
Referring to FIG. 3, an operating frequency of a waveform is a resonant frequency decided by a resonance between the leakage inductance Lr and the capacitor Cr. Since the magnetizing inductance Lm is relatively small, there is a considerable amount of the magnetizing current Im, which circulates a primary coil of the transformer regardless of power transmission. A current Ip of the primary coil is the sum of the magnetizing current Im and a secondary current, which is referred to by the primary coil.
The current Ip is delayed with respect to a voltage Vd which is applied to the resonant part 13, and accordingly, when the MOSFET, i.e., switches Q1 and Q2 are turned on, they achieve a zero voltage switching (ZVS). That is, when a voltage of the opposite ends of the MOSFET becomes a zero potential by a current flowing through an internal diode, the switches Q1 and Q2 are turned on.
FIGS. 4 and 5 illustrate changes in waveforms depending on a normal (inductive) mode and an overload (capacitive) mode of the LLC resonant converter in FIG. 2.
An input impedance of a resonant terminal is inductive under normal operation conditions. As shown in FIG. 4, the input current Ip of the resonant terminal is delayed with respect to the voltage Vd applied to the resonant terminal. This means that the MOSFET Q1 and Q2 operate under soft switching, i.e., under ZVS under which the MOSFET Q1 and Q2 are turned on at a zero voltage as in FIG. 4.
Under serious overload conditions, the input impedance of the resonant terminal is capacitive, and the input current Ip passes the voltage Vd as shown in FIG. 5. If the resonant converter operates in a capacitive mode, the MOSFET Q1 and Q2 do not achieve the ZVS and thus a high switching loss occurs due to a hard switching of the MOSFET Q1 and Q2. As a body diode of the MOSFET is reversely recovered during a switching conversion, a spike current and serious noise may occur. Accordingly, if the resonant converter operates in the capacitive mode, the MOSFET may be easily damaged for the aforementioned reason.
Generally, the LLC resonant converter minimizes switching loss and maximizes its efficiency through ZVS operation. However, when the LLC resonant converter operates under overload conditions, i.e., operates in the capacitive mode, the operating frequency is minimized and a circuit suffers a hard fail. Therefore, there is a need to prevent the LLC resonant converter from operating in the capacitive mode and to control the LLC resonant converter to perform a stable ZVS operation, to thereby ensure circuit stability.